Cathode construction



J 16. 1 5- R. J. BONDLEY CATHODE CONSTRUCTION Filed Dec. 8, 1941 2 Sheets-Sheet 1 Inventor: Ralph J. Bohd-ley. by W 5 M4 His Attorney Jan. 16, 1945. R, B ND LEY 2,367,331

CATHODE CONSTRUCTION Filed Dec. 8, 1941 2 Sheets-Sheet 2 FigZa "11/! IIIIIIIIIIIIIIIIIIII/ll Fig. 4.

Inventor: Ralph J.Bondley,

by Maw His Attorney.

Patented Jan. 16, 1945 CATHODE CONSTRUCTION Ra ph J. Bondley, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application December 8, 1941, Serial No. 422,054

8 Claims. ('Cl. 250-275) high frequency use, it is highly desirable that the spacing between the active surface of the cathode and the grid of the tube be not materially affected by thermal expansion of the parts of the cathode structure. This object is realized in accordance with the present invention by the employment of a mounting arrangement which assures that any motion of the emitting component of the cathode due to thermal expansion effects shall be along lines parallel to the plane of the grid-electrode. In a specific embodiment the mounting includes supporting elements which engage the emitting member in such fashion that thermal expansion of the elements is relieved by rotation of the member about an axis which is perpendicular, to the grid plane.

A further important aspect of the invention consists in the use of cathode supporting elements which are'so constructed and arranged as to perform the additional function of minimizing heat loss from the cathode emitter due to edgewise radiation effects.

The features desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following disclosure taken in connection with the drawings in which Fig. 1 is a longitudinal sectional view of a three-element vacuum tube suitably embodying the invention; Fig. 1a is a plan view of the grid of this tube; Fig. 2 is an enlarged and further sectionalized view of the cathode structure of Fig. 1; Fig. 2a is a detail of a portion of the cathode structure; Fig. 3 is a top view of the cathode as represented in Fig. 2; Fig. 4 is a view on an enlarged scale of a fragmentary portion of the structure of Fig. 3, and Fig. 5 is a fragmentary view of the upper portion of the cathode structure of Fig. 2 as it appears in its external aspects.

Referring particularly to Fig. 1 there is shown a high frequency triode having-an enclosing envelope constituted of a pair of glass cylinders I and H. The upper and lower extremities of the envelope are closed by means of circular metal members l3 and H which are sealed to the corresponding extremities of the parts It) and II,

both closure members being reinforced by securing to them stiffening plates 15 and I5 respectively. A central sealed joint for the envelope is provided by flanged rings l6 and I! which are respectively sealed to the cylinders l0 and II and which are welded to the opposite sides of an annular disk 19 projecting laterally beyond the normal contour of the tube.

Within the tube envelope and at its upper end there is provided an anode in the form of a hollow metal cylinder 22 having a lower surface 23 which is of planar character. The anode is supported by a connection to a heavy metal sleeve or bushing 25 which extends outwardly through the closure member l3 at its central region and which is hermetically joined to that member as by welding. The bushing 25 is externally threaded as indicated at 28 so as to facilitate its coning its fabrication. A thin-walled metal tube 33 which is sealed-in the upperextremity of the duct 29 provides convenient means for sealing off the tube after the envelope is evacuated, the sealing off step being accomplished either by pinch welding the tube 33 or by adding to it a glass tip which is sealed by fusing the glass.

The annular ring l9 has a central opening 36 which is or approximately the same dimensions as the lower surface 23 of the anode so that a discharge to the anode may occur through it. To permit the discharge to be controlled, grid wires 31, arranged as indicated in Fig. 1a, are secured to the lower surface or the ring l9 so as to occupy the opening 36. As clearly appears in Fig. 1a, these wires are of angularly bent configuration and are arranged so that the bent portions of the various wires are directed toward the center of the grid assembly. This arrangement is found to al'ford a substantial degree of freedom from changes in the control characteristic of the tube due to thermal expansion of the due to such expansion, this latter result being a consequence of the relative freedom of motion of the individual wires. The grid structure disclosed above is claimed and more fully described in my copending application Serial No. 567,367, filed December 9, 1944 entitled Electric discharge device and assigned to the assignee of the above invention.

The lower portion of the tube envelope contains a cathode structure having as its principal emitting element a circular plate or disk 40 consisting of tungsten, molybdenum or tantalum coated with an activating material such as thorium oxide. The plate 40 is supported from a thin metal cylinder 4| which constitutes an enclosure for the cathode structure as a whole, the details of the supporting means being described at a later point. The cylinder 4| is mounted on the closure member H by means of a flanged ring 42.

As appears more clearly in Fig. 2, there is provided in cooperative relation with the lower surface of the cathode disk 40 an auxiliary emitter in the form of a spirally arranged filament 45 (see Fig. 2a). This filament, which may be constituted of uncoated tungsten or of tungsten coated with an activating material such as thorium oxide is supported at its extremities by means of a pair of vertically arranged conductive rods 45 and 49. These merge at their lower ends with larger conductors and 52 which provide current supply connections for the filament. To permit the filament current to be introduced into the vacuum enclosure, the conductors 5| and 52 are associated at their lower extremities with lead-in seals by which they are connected to external terminals 54 and 55. In the arrangement illustrated, the lead-in seals include glass sleeves 55 and 51 respectively, these being fused to appropriately formed metal cylinders 58 and 59 which are Joined by welding to the reinforcing plate and to the closure member I4. Metal thimbles BI and 62 to which the conductors 5| and 52 are respectively joined, close the lower extremities of the lead-in seals, the terminals 54 and 55 being afiixed externally to the thimble surfaces. Upon passage of heating current through the filament (i. e., by means of the conductors 5| and 52) and upon application of potential between the filament and the main cathode emitter 40, a bombarding discharge may be produced to the latter element of sufiicient intensity to maintain it at an emitting temperature (say at 1600 degrees C.).

In order to minimize heat loss from the cathode structure, there is provided in connection with it auxiliary shielding means including a cup-shaped metal member 65. This member, which also serves to direct the emission from the filament 45 against the cathode part 40, is provided with a bottom part in the form of a flanged metal disk and is supported from the conductors 5| and 52 by means of quartz insulators 59 and 10. In order to clamp the shielding cup 55 to the quartz insulators 59 and 10, there is provided a channelshaped bracket II which straddles the insulators and is secured to the bottom of the shielding structure. The position of the insulators with respect to the conductors 5| and 52 is fixed by means of metal rings 12 welded or otherwise secured to the conductors. The degree of heat shielding, especially with respect to heat radiated from the lower surface of the cathode part 40, is still further increased by the use of metal disks l5 and 14 which are arranged transversely to the axis of the tube and which are supported from the bottom of the shielding cup 55 by means of a metal rod TI. Metal eyelets I9 serve to connect the rod 11 in rigid fashion to the shielding disks and the upper end of the rod provides a support for the central portion of the filament 46. With the arrangement described the entire shielding structure is maintained at the potential of the center point of the filament.

In a tube of the type under consideration it is highly desirable that the spacing between the active surface of the cathode part 40 and the grid structure shall remain constant in spite of the thermal expansion which inevitably accompanies heating of the cathode from room temperature to emitting temperature. This is especially important in the application of the tube to high frequency use in which any change in the spacing of the principal electrodes produces marked changes in the inter-electrode capacity and consequently in the operating characteristics of the tube. In order to assure constant spacing, the cathode part 40 is supported by means of a plurality of thin metal strips which extend between its peripheral edge and the surrounding cylinder 4|. As appears most clearly in Fig. 3, the supporting elements 90 are not directed radially but are at least partially tangential to the periphery of the part 40. To make this arrangement possible, the inwardly directed end of each of the supporting elements is bent in such a fashion that it makes good contact with the edge of the part 4|) and can 'be welded to it. The outer end of each element passes through a slot 93 provided in the upper edge of the cylinder 4| and is welded to the outer surface of the cylinder as indicated at 94 (see Fig. 4).

As a result of the disposition of parts indicated in Fig. 3, any expansion of the parts 90 which occurs as a result of heating is relieved by a counter-clockwise rotation of the cathode disk 40 in its plane of support. Consequently, there is no tendency for the supporting elements to becomebowed or distorted in vertical planes or to permit any vertical displacement of the part 40. The fact that the supporting elements 90 are of substantial width in a vertical direction gives them substantial rigidity in this direction and, therefore, minimizes their tendency to sag. At the same time the relative thinness of the elements prevents their serving as thermal conductors by which any substantial amount of heat can be carried from the part 40 to the surrounding cylinder 4|. In addition, the fact that the elements 90 present relatively extensive lateral surfaces to the edge of the part 40 causes them to serve as heat shields tending to minimize loss of heat from the part 4|) by radiation from its edge. These considerations, taken in conjunction with the other shielding means previously described, make it possible to maintain the emitting surfaces of the part 40 at a relatively very high temperature without excessive power input to the cathode heating system (i. e., to the bombarding discharge between the auxiliary filament 46 and the lower surface of the part 40).

From the standpoint of application of the tube of Fig. l to a high frequency system, for example, a system of the type described in McArthur application Serial No. 408,363, filed August 26, 1941, the supporting arrangement above described is advantageous in that it assures that the flow of high frequency current to the cathode part 4|) shall be symmetrical in a radial sense. This symmetry of current flow is further enhanced by the capacity which exists between the peripheral edge of the part 40 and the surrounding cylinder 4| and which assures a low impedance path for currents of radio frequency. (In the preferred mode 01' use of the tube the radio frequency circuit includes a connection to the bottom closure member l4 and thence through the cylinder 4| and the supporting elements 90 to the part 40) It is apparent that the supporting elements 90 may assume other forms than that illustrated. For example, instead of being constructed as thin strips, these elements may alternatively be made in the form of relatively fine wires or rods. All such elements, whether of symmetrical or flattened cross-section, I choose to describe as being of filamentary character and this term as used herein and in the appended claims should therefore be interpreted in accordance with this usage.

Moreover, while the invention has been described by reference to a particular embodiment, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cathode including a metal part adapted to be heated to a temperature of effective electron emission and means for mounting the said part so as to avoid displacement of the active surface of said part in a direction normal to said surface, said means consisting of a supporting structure effective to confine all motion of the.

said part resulting from, thermal expansion of the structure to planes transverse to said normal direction.

2. A cathode including a metal part adapted to be heated to a temperature of effective electron emission and means for mounting the said part so as to avoid displacement of the active surface of the .part in a direction normal to said surface, said means consisting of a supporting structure effective to confine all motion of the part resulting from thermal expansion of the structure to rotation about an axis parallel to the said normal direction.

3. In combination, a discharge device including an anode, a generally planar grid, a cathode having a planar emitting surface arranged in closely spaced parallel relation to said grid, and means for avoiding an change in the spacing between the said emitting surface and the grid because of thermal expansion effects, said means consisting of a supporting structure effective to confine any motion of the said emitting surface resulting from expansion of the structure to rotary motion about an axis perpendicular to the plane of the said grid.

4. A cathode structure comprising a metal part adapted to be heated to a temperature of effective electron emission and means for supporting the said part in a given plane, said supporting means including a plurality of filamentary elements engaging the periphery of said part and directed at least partially tangentially thereto, whereby expansion of the elements is relieved by rotation of the said part in its plane of support.

5. A cathode comprising a metal plate adapted to be maintained at a temperature of effective electron emission, and a mounting structure for said plate including a plurality of thin metal strips engaging the periphery of the plate and directed at least partially tangentially thereto so that thermal expansion of said strips is relieved by rotation of said plate in its plane of support.

6. A cathode comprising a metal plate adapted when heated to serve as a source of electron emission, means for electronically bombarding the plate to heat it to an emitting temperature, and means for supporting the plate in such fashion as to minimize heat loss from it, said supporting means including a plurality of filementary elements engaging the periphery of the plate and extending at least partially tangentially to it so that thermal expansion of said elements is relieved by rotation of the said plate in its plane of support.

'7. A cathode comprising a metal plate adapted when heated to serve as a source of electron emission, means for heating the plate, a mounting frame surrounding the plate and spaced from its outer periphery, and a plurality of filamentary elements extending between the mounting frame and the plate for supporting the latter from the former, said elements being directed at least partially tangentially with respect to said plate whereby thermal expansion of the elements is relieved by rotation of the plate in its plane of support.

8. A cathode structure comprising a circular metal plate adapted to serve as a source of electron emission, an auxiliary emitter spaced from one surface of the said plate and adapted when appropriately energized to produce an electron discharge to the plate so as to maintain the plate at an emitting temperature, a hollow metallic cylinder surrounding the said plate and auxiliary emitter and substantially enclosing the discharge path between them, and a plurality of thin metal strips extending between the cylinder and the said plate for supporting the latter from the former, said strips being so disposed as to present their major surfaces to the edge of the plate, whereby heat radiated from said edge is reflected back to the plate.

RALPH J. BONDLEY. 

